1. Field of the Technology
The disclosure relates to the field of imaging, particularly photomagnetic imaging using laser heating and magnetic resonance imaging.
2. Description of the Prior Art
Over the last few decades, a variety of in vivo imaging techniques have been developed using a broad spectrum of technologies to obtain structural, functional, and molecular information about the tissue under investigation. One category of these techniques is optical imaging. Optical imaging works by shining laser light on tissue and then measuring the results. Optical imaging can provide functional information, such as hemoglobin concentration, and can visualize exogenous contrast agents as well as molecular and functional markers. In vivo optical imaging extends across a wide range of applications, from the organ to the cellular levels.
One type of optical imaging, diffuse optical tomography (DOT) works by measuring with optical fibers how the path of light is altered by absorption and scattering as it passes through tissue. DOT can penetrate up to 10 centimeters of tissue, but only offers low-resolution images (>5 mm) due to the highly scattering nature of tissue. DOT can provide absorption maps at multiple wavelengths and hence, functional information such as tissue endogenous chromophore or exogenous contrast agent distribution. Furthermore. DOT can serve as a powerful molecular imaging tool in its fluorescent mode by providing the concentration and lifetime maps of a fluorescent molecular probes.
Significant effort has been expended on developing multi-modality imaging techniques to improve the resolution of optical imaging beyond DOT. One intriguing method combines optical and ultrasound techniques, and is called photo-acoustic imaging. Photo-acoustic tomography (PAT) is a type of photo-acoustic imaging which utilizes a short-pulsed laser (˜10 ns) to very rapidly raise the temperature of tissue. PAT works by creating an ultrasound wave as a result of the thermoelastic expansion of the tissue caused by the rise in temperature. This ultrasound wave is then detected by ultrasound transducers. Because the ultrasound waves scatter much less than optical waves in biological tissue, the overall resolution of the resulting image is improved dramatically. PAT can provide the same functional information as DOT, but with much higher resolution (˜1 mm). However, this is achieved at the cost of tissue depth penetration, as PAT is only capable of penetrating about 3 centimeters of tissue.
A recently developed type of photoacoustic imaging, photo-acoustic microscopy (PAM), modifies PAT by using high frequency ultrasonic transducers. PAM captures even higher resolution images (˜10 μm) than can be obtained with PAT, but the improved resolution comes at a much greater limitation on the imaging depth, as PAM can only penetrate to a depth of about 3 millimeters of tissue.
All the techniques mentioned above have a common weakness, in that they are only capable of acquiring data from the boundary of the medium. In the above mentioned techniques, two dimensional projection data is collected from many views to construct three dimensional images. The ultimate aims of these tomographic imaging techniques are to estimate the unknown distribution of optical properties in the whole volume imaged by using a finite number of measurements taken from the boundaries of the volume. This is achieved by solving an inverse problem, a framework used to convert observations and measurements into good estimations of physical properties—in this case, the physical properties of a volume of tissue. This problem is difficult to solving using both DOT and photoacoustic imaging techniques, because the number of unknown factors dwarfs the number of measurements that can be taken. Generally, however, photoacoustic imaging has been found to be superior to DOT, because of its utilization of ultrasound waves instead of optical photons
The first step of analyzing photoacoustic imaging data is to reconstruct the acoustic pressure source map. During this process, the medium is generally assumed to be acoustically homogeneous, and tissue heterogeneity may reduce the quantitative accuracy in this step. The second step is to convert the pressure map into an absorbed energy map. The third and the final step is to reconstruct the absorption and scattering maps. The greatest difficulty faced by this process is overcoming the fact that the absorbed energy at any particular point is a function of not only the absorption properties of the tissue, but also the local light fluence. The light fluence itself depends on both the light absorption and the light scattering properties of the tissue. Recently, there have been several different attempts to solve this challenging step.
Another major problem with photoacoustic imaging comes from the light distribution within tissue. The optical fluence decreases drastically as the photons travels from the illumination point at the boundary into areas deeper into the tissue. Because of this, noise-generating optoacoustic (OA) sources are predominantly created by the optical energy absorbed within the first centimeter of thicker tissues. These OA sources dramatically expand the dynamic range of the OA signals, thereby reducing the resolution and contrast of the image. Reflections of the ultrasonic waves generated by strong OA sources from the acoustic boundaries, such as echogenic areas inside the tissue, physical tissue boundaries, and the housing of piezoelectric transducers, can all be a source of acoustic noise. This noise not only creates artifacts in resulting images, but also reduces the ability of photoacoustic imaging systems to detect small lesions.
Another limitation of photoacoustic imaging is revealed by acoustic impedance mismatches between different tissue types, such as between bones or skull and the surrounding soft tissues.
A further limitation of photoacoustic imaging is that it requires a physical conducting medium in contact with and between the ultrasound transducers and the tissue under examination. Usually, this requires immersion of the tissue in a liquid, such as water. This creates practical difficulties, such as requiring test animals such as mice to be immersed in a tank in a hanging position, resulting in shifting of internal organs and changes to the physiological characteristics of the animal.
One of the primary applications of photoacoustic imaging is breast imaging for detection of tumor masses. Several systems for photoacoustic imaging of the breast have been reported in prior art. One such system, the laser optoacoustic imaging system (LOIS), has been estimated to be sufficient for single-pulse imaging of 6 to 11 mm tumors throughout the whole imaging slice of the breast. However, visualization of smaller masses still remains elusive.